Increased hydropower but with an elevated risk of reservoir operations in India under the warming climate

Summary Hydropower is a significant contributor to clean global electricity generation; therefore, it plays a crucial role in climate change mitigation. Notwithstanding major hydropower dams in India are in diverse climatic regions and exposed to risks because of the warming climate, potential changes in hydroclimate remain largely unexplored. Using observations and climate projections, we demonstrate the hydroclimatic changes in the upstream catchments and their implications for the hydropower generation of 46 major hydropower dams in India. A warmer (up to 5.0°C) and wetter projected climate with a substantial increase (5.0–33%) in precipitation will lead to an increased (7–70%) inflow to reservoirs of major dams. Increased inflow will enhance (9–36%) the hydropower production for most dams in the future, with a more prominent rise in central India dams. A simultaneous rise in extreme inflow and high reservoir storage conditions is projected under future climate for most dams. However, future climate changes project a favorable hydroclimate for hydropower production, with the associated risks related to extremes.


INTRODUCTION
The energy sector will likely play a crucial role in climate mitigation and the net-zero target. 1 India is the third largest energy-consuming nation globally, with a per capita energy consumption of 1208 kWh in 2020. 2 About 51% of energy in India is generated from traditional energy sources, 3 which emit greenhouse gases (GHG). Sustainable energy (wind, solar, and hydropower) can help mitigate climate change. 4 However, hydropower, a significant source of sustainable energy, has received considerable attention for its role in climate change mitigation. [5][6][7] Hydropower accounts for the second-largest share (13%) of the total energy produced in India, 8 and dominates the sustainable energy share.
Climate change can significantly alter global precipitation patterns and water resource availability. [9][10][11] Streamflow variability across the major global river basins is projected to change because of a warming climate. 12,13 Hydropower is susceptible to a warming climate 14,15 because of changes in reservoir storage and streamflow. 16 Thus, climate change can significantly alter the global hydropower generation capacity. [17][18][19] Indian sub-continental river basins (Ganga and Indus) are sensitive to a warming climate. 9,20 However, the impacts of climate change on India's hydropower potential remain unrecognized. Ali et al. 21 reported a significant increase in the hydropower potential of seven hydroelectric dams in India compared to the baseline period based on the CMIP5 projections. Therefore, the impact of future climate on the currently installed hydropower systems needs to be examined to stabilize the electricity supply and proper functioning of power plants in the future.
Global hydropower potential under climate change has been widely examined. [17][18][19] However, a regional assessment of hydropower potential in India under climate change remains limited. 21,22 Ali et al. 21 focused only on a small number of hydroelectric dams and did not examine the regional impacts of future climate on hydropower systems. The impacts of climate change on streamflow and hydropower potential may vary across regions in India, which remain unexplored. In addition, alternative risks to projected changes in hydropower potential have not been examined. For instance, extreme precipitation and floods have increased in the past 23,24 ; however, it remains unclear if the increase in precipitation can directly translate to an increase in hydropower potential without posing challenges to reservoir operations. Using observed Figure 1. Location of hydropower dams in different river basins in India Map shows the location of selected streamflow gauging stations (cyan) and large hydropower dams (red) in 13 Indian sub-continent river basins (white circles). The black and gray lines represent basin boundaries and major streamlines in the river basins, respectively. Shaded color represents the elevation (m) from mean sea level. Blue boxes show dams in North, Central, and South India.  Table S1]. Because the selected dams are large in terms of their installed hydropower capacity and located across the country, these can provide information related to regional variability in the changes in hydropower and inflow. The catchment area upstream of the dams varies between 700 and 220,000 km 2 . At the same time, the installed hydropower capacity (IHC) of the dams ranges between 33 and 1670 MW ( Figure 2). Bhakra Nangal, Naptha Jhakhari, Karcham, Sardar Sarovar, Nagarjuna Sagar, and Srisailam have higher hydropower potential ( Figure S1B). Dams in Mahanadi, Narmada, and Krishna river basins receive higher inflow ( Figure S1A). The mean inflow and mean hydropower potential for most dams are less than 750 m 3 /s and 550 MW, respectively (Figures S1C and S1D).
Project changes in precipitation and temperature in the upstream catchment of the major dams can influence streamflow variability, 20,25 which can alter hydropower potential. We estimated multimodel ensemble mean changes in precipitation and temperature for upstream catchments of hydroelectric dams across India using the bias-corrected projections from CMIP6-GCMs (see STAR Methods for more details). The ensemble mean changes were estimated for the near (2021-2040), mid (2041-2060), and far (2081-2100) periods against the reference period (1995-2014) for the two scenarios (SSP1-2.6 and SSP5-8.5).
Catchments upstream of all the selected dams are projected to experience substantial warming in the future under both scenarios ( Figure S2-S4). Compared to Central and South India, North India is iScience Article projected to experience higher warming in the future. [26][27][28] The highest warming ($5 C) is projected in North India, whereas the projected warming for central and south India is around 3-4 C ( Figure S4D). The projected warming in the near period ranges between 0.5 and 1.0 C and 0.50 to 1.3 C for the SSP1-2.6 and SSP5-8.5 scenarios, respectively ( Figure S2, Table S2). Similarly, the mid-period warming ranges between 0.9 and 1.4 C and 1.5 to 2.5 C for the SSP1-2.6 and SSP5-8.5, respectively ( Figure S3, Table S2). The highest warming is projected under the SSP5-8.5 scenario for the far period ( Figure S4D), which ranges between 3.3 and 5.4 C. The three regions with the majority of hydropower dams have different warming levels projected ( Figures S2-S4), which can influence the hydrology of the upstream catchments.
Similar to the substantial warming, most catchments upstream of the hydropower dams are likely to witness increased precipitation in the projected climate ( Figures S5-S7). Among the three regions, North and Central India are projected to receive a higher increase in precipitation than southern India (Table S3). The projected changes in precipitation in the upstream catchment area of hydropower dams show considerable spatial variability within the central and southern Indian regions ( Figures S5-S7). Ensemble mean projected changes in rainfall vary between 1.5 and 26.2% under SSP1-2.6 and between À2.5 and 20.4% under SSP5-8.5 in the near period (Table S3). A more substantial increase in precipitation is projected in the mid and far periods under both scenarios (Table S3). Most catchments upstream of the dams are projected to witness substantial warming and increased precipitation, which can alter the flow regime with implications on inflow to the dams and hydropower production.

Projected changes in inflow to dams and hydropower
Next, we estimated the projected changes in inflow simulated from the H08 and CaMa-flood models (see STAR Methods for details, Figures3, S8 and S9). Inflow for most of the dam is projected to rise in the future climate attributable to the substantial increase in precipitation (Table S4). Substantial warming and increase in precipitation are projected in the future climate; however, precipitation is the dominating factor controlling streamflow variability in the Indian sub-continental river basins. 20,25,29 Hydropower dams in central India will receive higher inflow in the future than in northern and southern India ( Figure 3). However, inflow for a few hydropower dams in Ganga, Mahanadi, Brahmani, and West-coast River basins is projected to decline in the future (Figures 3, S8, and S9, and Table S4), which can be because of rise in atmospheric water demands in response to the considerable warming 16 compared to increase in precipitation. Ensemble mean projected changes in inflow range between À9.5 and 55.7% in the near period, À7.7 and 64.5% in the mid-period, and À1.8 and 135.0% in the far period (Table S4). Among the 46 hydroelectric dams, Ukai and Kadana dams are projected to experience a considerable increase in streamflow (more than 100%) in the future climate primarily because of rise in precipitation ( Figure S7).
The projected rise in precipitation and temperature results in enhanced inflow to reservoirs of the major hydropower dams in India which can influence hydropower production. We estimated the projected changes in DHP for all 46 dams using the multimodel ensemble mean streamflow for the near, mid, and far periods (Figure 4, Tables S4 and S5). DHP projections are consistent with inflow projections for most dams, indicating a projected rise in hydropower potential in the future (Figures 4, S10, and S11). However, a few dams show a substantial intermodel uncertainty in inflow and DHP under the projected future climate (Tables S4 and S5). The projected change in DHP is the highest in the far period and the lowest for the near period ( Figures 4D and S10D). Projected change in DHP ranges between À6.2 and 39% in the near period,À24 and 58.2% in the mid-period, and À5.0 and 62.8% in the far period (Table S5). DHP is projected to rise by more than 50% in Tehri, Ramganga, Kadana, Omkareshwar, Maheshwar, and Sriramsagar dams in the far period ( Figure 4D). On the other hand, eight out of eleven dams in south India are projected to experience a decline in hydropower potential under SSP5-8.5 ( Figures S10 and S11). Overall, dams in central India are projected to experience a more substantial increase in DHP than in the other two (north and central) regions ( Figure 4).
Next, we estimated multimodel ensemble mean changes in inflow and DHP for different global warming levels (1.5 C, 2 C, 3 C) [ Figure S12 and  Figure S12). For instance, the median change in DHP for the dams in North, Central, and South India ranges between 10 and 20%, 20 and 40%, and À10 and 3% under the selected global warming levels. Consistent with the inflow, dams in south India are likely to witness a reduction in DHP at 1.5 C  Article and 3 C global warming levels ( Figure S12). The mean increase in DHP is the highest at a 3 C global warming level in all three regions. Based on the CMIP6 projections, we find a favorable condition for hydropower production for most of the dams in India under the warming climate, which can be attributed to a considerable rise in inflow to reservoirs. The increase in inflow is mainly because of a substantial increase  Table S1. For near and mid periods, see Figures S10 and S11.

Projected increase in high flow and risk to reservoir operations
We computed the projected changes in high inflow events under the warming climate for all the dams to examine the potential risks to reservoir operation ( Figure S13). The high-flow events were selected considering the 95 th percentile threshold for the historical period (1995-2014). The frequency of high-inflow events is projected to rise substantially for most hydropower dams in the future ( Figure S13). The highest increase in high inflow events to hydropower dams is projected in the far period under the highest emission scenario of SSP5-8.5 ( Figure S13). However, the frequency of high inflow events is projected to decline for the dams in south India in the near and mid periods ( Figure S13). The high-inflow events can pose challenges for the reservoir operations depending on the reservoir storage and magnitude of the events. The increased water availability during these high-flow events when the reservoir is full can pose challenges for controlling flooding in downstream regions. Most reservoirs receive significant storage during the summer monsoon season (June to September) after a considerable decline in storage in the pre-monsoon season to meet the irrigation demands. 33 Most of the dams in India are multi-purpose and can be used for hydropower, flood control, and irrigation water supply. The challenges for reservoir operations primarily arise during the peak monsoon season (July-August), when reservoirs are full and cannot accommodate the high inflow. 34 Next, we estimated the frequency of days when both reservoir storage and dam inflow exceed the 95 th percentile to examine the potential risk to reservoir operations. High inflow when reservoir storage is about full can pose challenges for reservoir operations. In the absence of reliable early warning and forecast systems, high inflow when reservoirs have already reached their total capacity may require quick release, which can cause flooding in the downstream regions. 35 In addition, the oversupply to reservoirs can also lead to risks of dam breaks. 36,37 Therefore, the rise in the high inflow events ( Figure S13) because of the heavy precipitation in the upstream catchment of the hydropower dam with the higher reservoir storage imposes risks to reservoir operations, causing the sudden opening of the dam gates and flooding in the downstream low-lying areas. 38 Indian sub-continental river basins during the observed period witnessed such extreme conditions in the past. For instance, the Machchhu dam failure in 1979 and the Kerala floods in 2018 were associated with reservoir storage. 39,40 Simultaneous occurrence of high inflow when reservoirs have high storage more often because of the warming climate (Figure 5). Reservoir operations can be challenging because of increased high inflow events, especially for hydropower dams in central India. On the other hand, hydropower dams in south India are projected to experience high reservoir storage and high inflow condition less frequently during the mid and far periods ( Figures 5D and 5E).

DISCUSSION
We estimated the changes in the hydropower potential across different regions of India under a warming climate. The climate change impacts on hydropower potential differ among regions, which project an increase in hydropower potential in North and Central India and a decline in hydropower potential for a few hydropower dams in South India. Hydropower production predominantly depends on precipitation and streamflow. 14 The increase in hydropower potential can be attributed to increased inflow in the future climate. Streamflow in North India river basins is greatly influenced by snow and glacial meltwater. Projected substantial warming in North India is likely to reduce snow and glacial storage, reducing snowmelt water contribution to the streamflow in the long run. 26,28,41 However, an increase in future inflow to reservoirs in North India is because of substantial increase in rainfall in the future climate. Furthermore, the projected decline in hydropower with increased precipitation for a few dams can be attributed to the higher evapotranspiration rate than precipitation in the future. 16 A more robust increase in inflow to dams is projected based on CMIP6-GCMs compared to the previous studies. 21,22 The projected increase in hydropower potential (up to 60%) is considerably higher than the previously reported [up to 25%]. 21 The increase in high-flow events because of rise in extreme precipitation is projected for India under future climate, which is consistent with the previous findings. 42 The projected increase in extreme inflow events will impose difficulties in the management of reservoirs for flood control, iScience Article hydropower production, and water supply. 43 In addition, the regional impacts of climate change on hydropower generation in India were not discussed in the previous studies. 21 Our results show considerable regional variability in the impact of climate change on hydropower production under the warming climate across India.
We used hydrological model simulations for the observed and projected future climate for runoff and streamflow. We find a considerable intermodal variation in the inflow and hydropower projections highlighting the uncertainty because of climate models (please see supplemental tables for further details). The ability of existing GCMs is limited in simulating the key features of the monsoonal climate because of coarser spatial resolution. Regional climate models at higher resolution that can better resolve topography can strengthen the projections. 44 The uncertainty associated with the hydrological models can mainly be because of input datasets and model parametrization. 45,46 The ensemble projections from multiple hydrological models can reduce parametric uncertainty in the hydroclimatic projections. 29,47 Other factors like land use/land cover change and construction of new reservoirs upstream of the existing hydropower dams can also impact river flow regimes and sediment load to the reservoirs, [48][49][50][51] which can influence hydropower potential in the future. The projected increase in high-flow events can increase sedimentation in the reservoir, which can further reduce the storage capacity. 52 We used a simplified reservoir operating algorithm for computing release that does not account for future irrigation and other demands, which can also affect hydropower production. 53 Despite these iScience Article uncertainties and limitations, our findings provide crucial information regarding the future changes in hydropower potential and water resources in the Indian sub-continental river basins.
Projected warmer and wetter conditions will increase most dams' hydropower potential. However, the impact of future climate on hydropower is not uniform across the dams. For instance, our results indicate that a decline in hydropower potential for a few hydropower dams in south India is projected. Dams in Tapi, Narmada, Mahi, Godavari, and Mahanadi river basins show a more remarkable rise in future hydropower potential. Based on selected hydroelectric dams, India is projected to increase hydropower potential (by 10-23%). However, the frequency of high inflow events is projected to rise for most hydropower dams, with the frequency of high reservoir storage. Reservoirs with high storage are projected to witness extreme inflow conditions more often in the future, leading to challenges for reservoir operations. Our findings can provide crucial insights related to projected changes in hydroclimate and hydropower for the major dams in India to planners and policymakers. In addition, we highlight the challenges and opportunities associated with climate change mitigation and adaptation in the context of hydropower in India under the warming climate. Our findings emphasize the need of reliable early warning systems that can assist reservoir operations in the future.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following:

AUTHOR CONTRIBUTIONS
V.M. designed the study. D.S.C. performed the analysis and wrote the initial draft. V.M. and D.S.C. worked on the discussion.

DECLARATION OF INTERESTS
Authors declare no competing interest.

INCLUSION AND DIVERSITY
We support inclusive, diverse, and equitable conduct of research. iScience Article provide us with information regarding the benefits of climate change mitigation. We bias-corrected the CMIP6 projections for the five GCMs to remove the bias using the method developed by Hempel et al., 57 which has been widely used in ISIMIP assessments. 21,58,59 The proposed bias correction method preserves the long-term climate signal in the GCM data. The bias correction was performed using observations from IMD for the Indian region and Sheffield et al. 54 for the region outside India. We estimated the percentage bias (p-bias) and normalized root-mean-square error (NRMSE) between CMIP6 precipitation and observed precipitation data to examine the accuracy of the bias-correction method (Figures S15-S17, Table S8).